Light and Dark Adaptation
[Dark adaptation] [Factors affecting dark adaptation] [Light adaptation]
Dark Adaptation.
The eye operates over a large range of light levels. The sensitivity of our eye can be
measured by determining the absolute intensity threshold, that is, the minimum luminance of
a test spot required to produce a visual sensation. This can be measured by placing a subject in a dark
room, and increasing the luminance of the test spot until the subject reports its presence. Consequently,
dark adaptation refers to how the eye recovers its sensitivity in the dark following exposure to bright
lights. Aubert (1865) was the first to estimate the threshold stimulus of the eye in the dark by measuring
the electrical current required to render the glow on a platinum wire just visible. He found that the
sensitivity had increased 35 times after time in the dark, and also introduce for the term "adaptation".
Dark adaptation forms the basis of the Duplicity Theory which states that above a certain luminance
level (about 0.03 cd/m2), the cone mechanism is involved in mediating vision; photopic vision. Below
this level, the rod mechanism comes into play providing scotopic (night) vision. The range where two
mechanisms are working together is called the mesopic range, as there is not an abrupt transition
between the two mechanism.
The dark adaptation curve shown below depicts this duplex nature of our visual system (figure 1). The
first curve reflects the cone mechanism. The sensitivity of the rod pathway improves considerably after
5-10 minutes in the dark and is reflected by the second part of the dark adaptation curve. One way to
demonstrate that the rod mechanism takes over at low luminance level, is to observe the colour of the
stimuli. When the rod mechanism takes over, coloured test spots appear colourless, as only the cone
pathways encode colour. This duplex nature of vision will affect the dark adaptation curve in different
ways and is discussed below.
To produce a dark adaptation curve, subjects gaze at a pre-adapting light for about five minutes, then
absolute threshold is measured over time (figure 1). Pre-adaptation is important for normalisation and to
ensure a bi-phasic curve is obtained.
Figure 1. Dark adaptation curve. The shaded area represents 80% of the group of subjects. Hecht and Mandelbaum's data from From Pirenne M. H., Dark
Adaptation and Night Vision. Chapter 5. In: Davson, H. (ed), The Eye, vol 2. London, Academic Press, 1962.(6 K jpeg image)
From the above curve, it can be seen that initially there is a rapid decrease in threshold, then it declines
slowly. After 5 to 8 minutes, a second mechanism of vision comes into play, where there is another rapid
decrease in threshold, then an even slower decline. The curve asymptotes to a minimum (absolute
threshold) at about 10-5 cd/m2 after about forty minutes in the dark.
Factors Affecting Dark Adaptation.
1.
2.
3.
4.
Intensity and duration of the pre-adapting light
Size and position of the retinal are used in measuring dark adaptation
Wavelength distribution of the light used
Rhodopsin regeneration
Intensity and duration of pre-adapting light:
Different intensities and duration of the pre-adapting light will affect dark adaptation curve in a number
of areas. With increasing levels of pre-adapting luminances, the cone branch becomes longer while the
rod branch becomes more delayed. Absolute threshold also takes longer to reach. At low levels of preadapting luminances, rod threshold drops quickly to reach absolute threshold (figure 2).
Figure 2. Dark adaptation curves following different levels of pre-adapting luminances. Hecht, Haig and Chase's data from Bartlett N. R., Dark and Light
Adaptation. Chapter 8. In: Graham, C. H. (ed), Vision and Visual Perception. New York: John Wiley and Sons, Inc., 1965.(9 K jpeg image)
The shorter the duration of the pre-adapting light, the more rapid the decrease in dark adaptation (figure
3). For extremely short pre-adaptation periods, a single rod curve is obtained. It is only after long preadaptation that a bi-phasic, cone and rod branches are obtained.
Figure 3. Dark adaptation curves following different duration of a pre-adapting luminance. Wald and Clark's data from Bartlett N. R., Dark and Light Adaptation.
Chapter 8. In: Graham, C. H. (ed), Vision and Visual Perception. New York: John Wiley and Sons, Inc., 1965.(12 K jpeg image)
Size and location of the retina used:
The retinal location used to register the test spot during dark adaptation will affect the dark adaptation
curve due to the distribution of the rod and cones in the retinal (figure 4).
Figure 4. Distribution of rod and cones in the retina. Plot from Osterberg's data. (12 K jpeg image)
When a small test spot is located at the fovea (eccentricity of 0o ), only one branch is seen with a higher
threshold compared to the rod branch. When the same size test spot is used in the peripheral retina
during dark adaptation, the typical break appears in the curve representing the cone branch and the rod
branch (figure 5).
Figure 5. Dark adaptation measured with a 2o test spot at different angular distances from fixation.
Subject was pre-adapted for 2 minutes to 300 mL. Hecht, Haig and Wald's data from Bartlett N. R., Dark and Light Adaptation. Chapter 8. In: Graham, C. H. (ed),
Vision and Visual Perception. New York: John Wiley and Sons, Inc., 1965.(12 K jpeg image)
A similar principle applies when different size of the test spot is used.
When a small test spot is used during dark adaptation, a single branch
is found as only cones are present at the fovea. When a larger test spot
is used during dark adaptation, a rod-cone break would be present
since the test spot stimulates both cones and rods. As the test spot
becomes even larger, incorporating more rods, the sensitivity of the
eye in the dark is even greater (figure 6), reflecting the larger spatial
summation characteristics of the rod pathway.
Figure 6. Dark adaptation measured using different size test spots. Hecht, Haig and Wald's data from Bartlett N. R., Dark and Light Adaptation. Chapter 8. In:
Graham, C. H. (ed), Vision and Visual Perception. New York: John Wiley and Sons, Inc., 1965. (6K jpeg image)
Wavelength of the threshold light:
When stimuli of different wavelengths are used, the dark adaptation curve is affected. From figure 7
below, a rod-cone break is not seen when using light of long wavelengths such as extreme red. This
occurs due to rods and cones having similar sensitivities to light of long wavelengths (figure 8). Figure 8
depicts the photopic and scotopic spectral sensitivity functions to illustrate the point that the rod and cone
sensitivity difference is dependent upon test wavelength (although normalization of spatial, temporal and
equivalent adaptation level for the rod and cones is not present in this figure). On the other hand, when
light of short wavelength is used, the rod-cone break is most prominent as the rods are much more
sensitive than the cones to short wavelengths once the rods have dark adapted.
Figure 7. Dark adaptation curve using different test stimuli of different wavelengths. Subjects were pre-adapted to 2000mL for 5 minutes. A 3 degree test stimuli
was presented 7 degrees on the nasal retina. The colours were: RI (extreme red)=680 nm; RII (red)=635 nm; Y (yellow)=573 nm; G (green)=520 nm; V
(violet)=485nm and W (white). Chapanis' data from Bartlett N. R., Dark and Light Adaptation. (Chapter 8. In: Graham, C. H. (ed), Vision and Visual Perception.
New York: John Wiley and Sons, Inc., 1965).(8 K jpeg image)
Figure 8. Scotopic (rods) and photopic (cones) spectral sensitivity functions. Wald's data from Davson, H., Physiology of the Eye, 5th ed. London: Macmillan
Academic and Professional Ltd, 1990.(8 K jpeg image)
Rhodopsin regeneration
Dark adaptation also depends upon photopigment bleaching. Retinal (or reflection) densitometry, which
is a procedure based on measuring the light reflected from the fundus of the eye, can be used to
determine the amount of photopigment bleached. Using retinal densitometry, it was found that the time
course for dark adaptation and rhodopsin regeneration was the same. However, this does not fully
explain the large increase in sensitivity with time. Bleaching rhodopsin by 1% raises threshold by 10
(decreases sensitivity by 10). In figure 9, it can be seen that, bleaching 50% of rhodopsin in rods raises
threshold by 10 log units while the bleaching 50% of cone photopigment raises threshold by about one
and a half log units. Therefore, rod sensitivity is not fully accounted for at the receptor level and may be
explained by further retinal processing. The important thing to note is that bleaching of cone
photopigment has a smaller effect on cone thresholds.
Figure 9. Log relative threshold as a function of the percentage of photopigment bleached.
From Cornsweet, T.N., Visual Perception. New York: Academic Press, 1970.(6 K jpeg image)
Light Adaptation.
With dark adaptation, we noticed that there is progressive decrease in threshold (increase in
sensitivity) with time in the dark. With light adaptation, the eye has to quickly adapt to the
background illumination to be able to distinguish objects in this background. Light adaptation can be
explored by determining increment thresholds. In an increment threshold experiment, a test stimulus is
presented on a background of a certain luminance. The stimulus is increased in luminance until detection
threshold is reached against the background (figure 10) Therefore, the independent variable is the
luminance of the background and the dependent variable is the threshold intensity or luminance of the
incremental test required for detection. Such an approach is used when visual fields are measured in
clinical practice.
Figure 10. Light adaptation using an increment threshold experiment. (a) example of the stimulus used (b) luminance profile of the stimulus.(5K jpeg image)
The experimental conditions shown in figure 10, can be repeated by changing the background field
luminance. Depending upon the choice of test and background wavelength, the test size and retinal
eccentricity, a monophasic or biphasic threshold versus intensity (tvi) curve is obtained. Figure 11
illustrates such a curve for parafoveal presentation of a yellow test field on a green background. This
stimulus choice leads to two branches. A lower branch belonging to the rod system. As the background
light level increases, visual function shifts from the rod system to the cone system. A dual-branched
curve reflects the duplex nature of vision, similar to the bi-phasic response in the dark adaptation curve.
Figure 11. Light adaptation curve plotted as increment threshold versus background luminance (or a threshold-versus-intensity: tvi curve). The above plot shows
increment threshold (Nl ) and background luminance (Mm ). Light of two different wavelengths are used in this case (580 nm for the test and 500 nm for the
background). Stiles' data from Davson (Davson's Physiology of the Eye, 5th ed. London: Macmillan Academic and Professional Ltd, 1990).(6K jpeg image)
When a single system (eg. the rod system) is isolated under certain experimental conditions, four
sections of the curve is apparent. These experiment conditions involve using a red background to
suppress the cone photoreceptors and a green test spot to stimulate the rod photoreceptors (Aguilar and
Stiles, 1954). The curve in figure 12 can also be obtained by performing increment threshold experiments
on rod monochromats who lack cone photoreceptors. When the rod system is isolated using the
conditions of Aguilar and Stiles, four sections are obtained:
a.
b.
c.
d.
Dark light
Square Root Law (de Vries-Rose Law)
Weber's Law
Saturation
The threshold in the linear portion of the tvi curve is determined by the dark light level. As background
luminance is increased, the curve remains constant (and equal to the absolute threshold). Sensitivity in
this section is limited by neural (internal) noise, the so called "dark light". The background field is
relatively low and does not significantly affect threshold. This neural noise is internal to the retina and
examples of these include thermal isomerisations of photopigment, spontaneous opening of
photoreceptor membrane channels and spontaneous neurotransmitter release.
Figure 12. Schematic of the increment threshold curve of the rod system. Aguilar and Stiles' data from Davson (Davson's Physiology of the Eye, 5th ed. London:
Macmillan Academic and Professional Ltd, 1990).
The second part of the tvi curve is called the square root law or (de Vries-Rose Law) region. This part
of the curve is limited by quantal fluctuation in the background. Rose (1948) proposed that visual
threshold would be quantal limited. The visual system is usually compared to a theoretical construct, an
ideal light detector. An ideal detector can detect and encode each absorbed quantum of light and is
limited only by the noise due to quantal fluctuations in the source. To detect the stimulus, the stimulus
must be sufficiently exceed the fluctuations of the background (noise).
Because the variability in quanta increases with the number of quanta absorbed, threshold would increase
with background luminance. In fact, the increase in threshold should be proportional to the square root of
the background luminance; hence the slope of one half in a log-log plot. For the rod pathway a slope of
0.6 is often found (Hallett, 1969). Barlow (1958) explored the conditions which influenced the transition
from the square root law to Weber’s law (see below). He concluded that for brief, small test spots,
increment thresholds rise as the square root of the background over the entire photopic range. Spots of
large areas and long durations have slopes close to Weber's law. Other spatio-temporal configurations
result in different proportions of each region.
When plotted using log DL versus log L coordinates, the Weber law section ideally has a slope 1. For
the rod pathway, a slope 0.8 or less is found, implying that the rod pathway does operate under true
Weber conditions. This section of the curve demonstrates an important aspect of our visual system. Our
visual system is designed to distinguish objects from its background. In the real world, objects have
contrast, which is constant and independent of ambient luminance. Therefore, the principle of Weber’s
law can be applied to contrast which remains constant regardless of illumination changes. This is called
contrast constancy or contrast invariance, with this contrast level defined as Webers constant. Contrast
constancy can be mathematically expressed as DL /L = constant. DL is the increment threshold on a
background L. The constant is also known as the Weber constant or Weber fraction. The Weber constant
for the rod and cone is 0.14 (Cornsweet, 1970) and 0.02 to 0.03 (Davson, 1990) respectively. Within the
cone pathways, the S-cone pathway, again has a different characteristics to those of the longerwavelength pathway with a Weber constant of around 0.09 (Stiles, 1959).
Section 4 of the curve (figure 12) shows rod saturation at high background luminance. The slope begins
to increase rapidly and the rod system starts to become unable to detect the stimulus. This section of the
curve occurs for cone mechanism under high background light levels.
References.
Aguilar M and Stiles WS (1954) Saturation of the rod mechanism of the reina at high
levels of stimulation. Optica Acta. 1: 59-65.
Barlow HB (1958) Increment thresholds at low intensities considered as signal noise
discriminations. J Physiol 141:337-350.
Barlow HB (1964) The physical limits of visual discrimination. In: Giese A. C. (ed),
Photophysiology. Chapter 16. New York: Academic Press.
Bartlett NR (1965) Dark and Light Adaptation. Chapter 8. In: Graham, C. H. (ed), Vision and
Visual Perception. New York: John Wiley and Sons, Inc.
Cornsweet, TN (1970) Visual Perception. New York: Academic Press.
Davson H (1990) Physiology of the Eye, 5th ed. London: Macmillan Academic and Professional
Ltd.
Hallett PE (1969) The variations in visual threshold measurement. J Physiol 202:403-419
Pirenne MH (1962) Dark Adaptation and Night Vision. Chapter 5. In: Davson, H. (ed), The Eye,
vol 2. London, Academic Press.
Pirenne MH (1962) Rods and Cones. Chapter 2. In: Davson, H. (ed), The Eye, vol 2. London,
Academic Press.
Rose A (1948) The sensitivity performance of the human eye on a absolute scale. J Opt Soc Amer
38:196-208.
Stiles WS (1959) Colour vision: the approach through increment threshold sensitivity. Proc Nat
Acad Sci 75:100-114.
[Dark adaptation] [Factors affecting dark adaptation] [Light adaptation]